The present invention relates to an interface circuit suitable for use in connecting a tablet input device (hereinafter referred to as a "tablet") to a microcomputer such as a personal computer or a television gate device.
Most of conventional personal computers and television gate devices have a microcomputer analog input unit to which a multidirectionally operable variable resistor can be connected. The multidirectionally operable variable resistor has a lever movable in X and Y directions to perform various operation on the computer, such as a game.
FIG. 1 of the accompanying drawings is illustrative of such a multidirectionally operable variable resistor connected to an analog input unit of a microcomputer. A multidirectionally operable variable resistor 1 is connected to a power supply outlet terminal 3 and an input terminal 4 of an analog input unit 2 of a microcomputer. The analog input unit 2 comprises a capacitor 5, an on-off switch 6, an input terminal 7, a comparator 8, and an output terminal 9. The on-off switch 6 can be periodically closed to discharge the capacitor 5 intermittently. The comparator 8 serves to compare a voltage E.sub.1 across the capacitor 5 with a reference voltage E.sub.S1 supplied from the input terminal 7 and to issue a voltage E.sub.0 having alternate higher and lower levels. The variable resistor 1 has one terminal coupled to the power supply outlet terminal 3 and the other terminal to the input terminal 4.
Normally, the voltage supplied to the power supply outlet terminal 3 is 5 (V), and the reference voltage E.sub.S1 applied from the input terminal 7 is 2 (V).
FIG. 2 is a diagram showing the waveforms of voltages in the circuit arrangement of FIG. 1. Identical reference characters denote identical voltages in FIGS. 1 and 2.
With the voltages specified as above, operation of the circuit arrangement of FIG. 1 will be described with reference to FIG. 2.
When the multidirectionally operable variable resistor 1 is connected between the power supply outlet terminal 3 and the input terminal 4, the variable resistor 1 and the capacitor 5 jointly form a time-constant circuit. The on-off switch 6 is alternately turned on and off by a switching signal S of a fixed period, that is, the on-off switch 6 is closed for a time interval t in each period to discharge the capacitor 5.
The voltage E.sub.1 across the capacitor 5 falls to zero (V) during the time interval t in which the on-off switch 6 remains closed. At the same time that the on-off switch 6 is opened, the voltage E.sub.1 rises at a rate according to a time constant determined by the resistance R of the variable resistor 1 and the static capacitance C of the capacitor 5. The voltage E.sub.1 becomes 5.times.(1-e.sup.- CR) (V) which would reach 5 (V). When the on-off switch 6 is closed at a next time, the capacitor 5 is discharged through the on-off switch 6 to cause the voltage E.sub.1 to fall to zero (V).
The voltage E.sub.1 as it cyclically varies between zero (V) and 5 (V) is applied to the positive terminal of the comparator 8 in which the voltage E.sub.1 is compared with the reference voltage E.sub.S1 of 2 (V) which has been supplied from the input terminal 7 to the negative terminal of the comparator 8. The comparator 8 issues an output voltage E.sub.0 of high level when E.sub.1 .gtoreq.E.sub.S1 and an output voltage E.sub.0 of low level when E.sub.1 &lt;E.sub.S1. The output voltage E.sub.0 is supplied via the output terminal 9 to an analog-to-digital converter (not shown) from which a digital signal is delivered as data for the microcomputer.
The resistance R of the variable resistor 1 can be varied by manipulating a lever (not shown) attached to the variable resistor 1. The gradient of a rate of increase of the voltage E.sub.1 varies when the resistance R is changed. More specifically, the gradient of rise of the voltage E.sub.1 becomes smaller as the resistance R gets larger, and becomes steeper as the resistance R gets smaller. Therefore, the period in which the output voltage E.sub.0 from the comparator 8 remains lower in level varies with the resistance R. This period includes an interval in which the on-off switch 6 is closed, and a period of time T from which such an interval is precluded is representative of the resistance R of the multidirectionally operable variable resistor 1.
For this reason, the data indicative of the resistance R can be supplied to the microcomputer, and various operations such as a game can be performed by manipulating the lever of the multidirectionally operable variable resistor 1.
The lever of the multidirectionally operable variable resistor 1 is however actuatable only for changing the position of a predetermined image displayed on a display unit. The variable resistor 1 fails to provide a capability such as for studying equipment, that is, to enter pattern data of desired images and characters into a microcomputer and display the pattern data on the display.
One known expedient for achieving such a capability comprises a thin-film switch known as a tablet composed of a flat resistor layer and a conductive layer. The tablet is connected to the microcomputer, and the data of a pattern described by a stylus on the tablet surface is entered into the microcomputer. The tablet has been employed as a data input device for a microcomputer in industrial machines such as designing machines.
Since the output voltage from the tablet used in the industrial machines is an analog voltage, there is connected an interface circuit between the tablet and the microcomputer for converting the analog voltage into a digital voltage.
With the tablet connected to a personal computer or a television game device, any desired pattern data of images and characters can be entered into the microcomputer through the tablet and displayed on the display unit. However, interface circuits which have been heretofore available is highly expensive, and personal computers and television game devices have had no terminal for digital data entry, or for connection to the interface circuit.
Instead, the personal computers and television game devices presently available have analog input units for connection to the multidirectionally operable variable resistor. Since the tablet produces an analog output voltage, it would be possible to connect the tablet directly to the analog input unit.
In FIG. 1, the resistance R of the multidirectionally operable variable resistor 1 can vary widely in the range of from zero (.OMEGA.) to 200 (k.OMEGA.) or 400 (k.OMEGA.), and the static capacitance C of the capacitor 5 is selected such that the gradient of rise of the voltage E.sub.1 can sufficiently be discriminated over that range. However, the resistance of the tablet varies in the range of from zero (.OMEGA.) to 300 (.OMEGA.) or 500 (.OMEGA.) which is much smaller than the resistance range of the variable resistor 1. Accordingly, any variation in the tablet resistance would cause a slight change in the gradient of rise of the voltage E.sub.1, thus failing to provide a sufficient resolution.
As described above, the personal computers and television game devices have heretofore required expensive interface circuits in order to connect a tablet. The tablet would fail to function sufficiently as desired if it were connected to the microcomputer without an interface circuit.